The present invention relates to an electrolytic capacitor used for various electronic apparatus, and particularly, to a surface-mounted type solid electrolytic capacitor having terminals for external connection, and a method of manufacturing same.
As conventional solid electrolytic capacitors having terminals for external connection, surface-mounted type tantalum solid electrolytic capacitors are representative ones, which are used for various electronic apparatus in large quantities. This type of tantalum solid electrolytic capacitor is described as an example in the following. The configuration of such conventional tantalum solid electrolytic capacitor includes a capacitor element portion and lead frame portion. As a material for the lead frame, nickel-base alloy or copper-base alloy is mainly used. Particularly, nickel-base alloy such as 42 alloy is used when the terminal of the lead frame portion is required to have a repeated bending strength or the terminal is required to have a mechanical strength enough to endure actual device installation. Also, copper-base alloy such as copper-nickel-tin alloy is used when the lead frame portion is required to have specially high processability.
A conventional tantalum solid electrolytic capacitor of this type will be described in the following with reference to FIG. 12 and FIG. 14.
FIG. 12 is a sectional view showing the configuration of a conventional tantalum solid electrolytic capacitor. In FIG. 12, the conventional tantalum solid electrolytic capacitor comprises a capacitor element 12 and positive electrode lead wire 13. The capacitor element 12 comprises a porous positive electrode body, and a dielectric oxide film layer, solid electrolytic layer and negative electrode layer (all of these are not shown) which are sequentially formed on the outer surface of the positive electrode body. The porous positive electrode body is formed by sintering a compact of tantalum powder. One end of the positive electrode lead wire 13 is exposed. One end of positive electrode terminal 14 is connected by welding or the like to the positive electrode lead wire 13 of the capacitor element 12. The other end of the positive electrode terminal 14 is led out of outer jacket resin 17 described later and is bent along the outer jacket resin 17. In this way, a terminal for external connection is formed. One end of negative electrode terminal 15 is connected to the negative electrode layer of the capacitor element 12 via conductive bonding agent 16, and the other end of the negative electrode terminal 15 is led out of the outer jacket resin 17 described later and is bent along the outer jacket resin 17. In this way, a terminal for external connection is formed. The capacitor element 12 is coated with the outer jacket resin 17 capable of electrical insulation so that each of the positive electrode terminal 14 and negative electrode terminal 15 is partially exposed to the outside.
FIG. 13 is a plan view showing the lead frame which forms the positive electrode terminal 14 and the negative electrode terminal 15. FIG. 14 is a sectional view of the line 14Axe2x80x9414A portion of FIG. 13. In FIG. 13 and FIG. 14, lead frame 18 is formed of a strip-form metallic member made of nickel-base alloy (such as 42 alloy) or copper-base alloy (such as copper-nickel-tin alloy). The positive electrode terminal 14 and the negative electrode terminal 15 are formed at the lead frame 18. Silver-plated layer 20 is disposed at element fixed portion 19. Guide hole 21 for transport is formed in the lead frame 18. As an undercoat layer 22, a copper or copper alloy plated layer of 0.3 xcexcm in thickness is disposed on the positive electrode terminal 14 and the negative electrode terminal 15. For soldering in actual device installation, plated layer 23 for soldering, formed from tin or tin-lead alloy, is disposed on the undercoat layer 22.
Next, a method of manufacturing a conventional tantalum solid electrolytic capacitor having such configuration as described above.
First, the capacitor element 12 is disposed at element fixed portion 19 located between the oppositely disposed positive electrode terminal 14 and negative electrode terminal 15 extending from the lead frame 18. The positive electrode lead wire 13 of the capacitor element 12 is connected by welding or the like to the positive electrode terminal 14 formed at the lead frame 18. The negative electrode layer of the capacitor element 12 is bonded by conductive bonding agent 16 of silver paste to silver plated layer 20 disposed on the negative electrode terminal 15 formed at the lead frame 18. The conductive bonding agent 16 is hardened under heat to establish electrical connection. The bonding agent 16 is heated at temperatures of 170xc2x0 C. to 180xc2x0 C. for about one hour for hardening.
Next, with each of the positive electrode terminal 14 and negative electrode terminal 15 partially exposed to the outside, the capacitor 12 is coated by outer jacket resin 17 capable of electrical insulation. Thus, the outer jacket resin 17 is heat-treated at 170xc2x0 C. to 180xc2x0 C. for about six hours for complete hardening. In this way, the outer jacket resin 17 is improved in cross-linking ability, and as a result, the tantalum solid electrolytic capacitor will be, for example, improved in moisture resistance. After that, thermal screening is performed in a furnace at 240xc2x0 C. to 260xc2x0 C. for about 60 sec. Thus, excessive current leakage or shorting trouble that may cause hindrance to the user during actual reflow soldering, for example, can be prevented from occurrence. After that, unnecessary portions of the lead frame 18 are removed, and thereafter, the characteristic and appearance inspections are performed before delivery of the finished product.
In the assembling process of a conventional tantalum solid electrolytic capacitor manufactured by such manufacturing method as described above, the capacitor will be subjected to severe heat history in the atmosphere. Therefore, the plated film is required to be heat resisting adhesion even after application of such heat history, and further, it is required to ensure excellent solder wettability, for example, during reflow soldering performed by the user.
Also, in this way, for making the plated film heat resistant and realizing excellent solder wettability, it is necessary for the copper or copper alloy plated layer disposed as the undercoat layer 22 for the above conventional positive electrode terminal 14 and negative electrode terminal 15 to have a thickness of 0.3 xcexcm at least. The thickness of the undercoat layer 22 is closely related to the heat resisting adhesion of the tin or tin-lead alloy plated layer as the plated layer 23 for soldering which is disposed on the undercoat layer 22. Also, the adhesion of the tin plated layer or tin-lead alloy plated layer depends upon the forming volume of thermal diffusion layer in connection with the copper or copper alloy plated layer that is the undercoat layer 22, and the copper or copper alloy plated layer acts to promote the formation of the thermal diffusion layer.
Thus, since the copper or copper alloy plated layer provided as the undercoat layer 22 contributes to heat resisting adhesion, the state of plated film is preferable to be very dense. To achieve the purpose, it is necessary to perform the plating under appropriate conditions with respect to the current density and plating bath control. Due to such severe conditions for plating bath control, it is possible to realize a plating thickness of 0.3 xcexcm or over. If such conditions are not satisfied, for example, performing the plating at excessive current density, then the copper or copper alloy plated layer formed will become porous. Accordingly, the layer is insufficient in adhesion. Also, even when the plated film of the copper or copper alloy plated layer is very dense, sufficient heat resisting adhesion cannot be obtained if the plating thickness is less than 0.3 xcexcm. The upper limit of the plating thickness for the above copper or copper alloy plated layer is not limited, but it is preferable to be about 4 xcexcm or less.
Accordingly, as a method of forming excellent plated film on a conventional lead frame 18, one of the following two methods has been employed.
The first method is such that when the strip-form metallic member used is made of nickel-base alloy or copper-base alloy, very dense undercoat layer 22 is formed on the strip-form metallic member in order to provide the solder or tin-plated film with a specified adhesion. The undercoat layer 22 is formed by nickel plating or copper plating, or by both of nickel plating and copper plating. Subsequently, silver plated layer 20 is formed in the form of stripes by electrolytic plating on a desired surface portion of negative electrode terminal 15 connected to the negative electrode layer of capacitor element 12 of the lead frame 18. The silver plated layer 20 thus formed is improved in conformability with the negative electrode terminal 15. On the entire surface and back of the lead frame 18 other than the silver plated layer 20 is formed a plated layer 23 for soldering which is formed from solder or tin plated film. After that, punching by a die is performed to make the positive electrode terminal 14 and negative electrode terminal 15. That is, in the first method, plated film is formed on a strip-form metallic member, followed by punching process to make the positive electrode terminal 14 and negative electrode terminal 15.
The second method is such that the strip-form metallic member is punched in advance to continuously make the positive electrode terminal 14 and negative electrode terminal 15 in the form of hoops on the lead frame 18, and after that, same processing as in the first method is performed in order to form plated film.
However, in the positive electrode terminal 14 and the negative electrode terminal 15 of the above conventional solid electrolytic capacitor, the undercoat layer is formed by nickel plating on the lead frame 18 formed of a strip-form metallic member. After that, when the lead frame is further tin- or solder-plated, an intermetallic compound layer is formed between tin and nickel due to severe heat history applied during the manufacturing process, thereby losing the surface tin or solder layer. As a result, there arises a problem of worsening of solder wettability. In order to solve this problem, it is necessary to form the tin-plated layer or solder-plated layer thicker enough so that excellent solder wettability can be obtained even after heat history. In this case, the cost of the product will become very high. Accordingly, it is difficult to employ the method from the industrial point of view.
Also, in the method of manufacturing the positive electrode terminal 14 and the negative electrode terminal 15, even when the lead frame 18 formed of a strip-form metallic member is plated with copper or nickel, and further plated with copper, followed by tin or solder plating, there is formed an intermetallic compound between tin and copper due to severe heat history. And it results in losing the surface tin layer or solder layer. Consequently, there arises a problem of worsening of the solder wettability. Also, there is a problem of peeling that is generated between the tin-copper intermetallic compound layer and the plated layer.
In order to solve these problems, the Japanese Laid-Open Patent H5-98464 proposes the following method. A copper undercoat layer of 0.1 xcexcm to 1.0 xcexcm in thickness is formed on the base material of the lead frame formed from nickel or nickel alloy, and further, tin or solder plated layer is formed on the copper undercoat layer. After that, reflow treatment or hot dipping of tin or solder is performed. Thus, a tin-copper intermetallic compound of 0.2 xcexcm to 2.0 xcexcm in thickness is formed between the base material formed from nickel or nickel alloy and the plated layer such as tin-plated layer or solder-plated layer.
Also, the Japanese Laid-Open Patent H6-196349 proposes the following method. A nickel undercoat layer is formed on the base material of the lead frame formed from copper or copper alloy, and further, a copper undercoat layer of 0.1 xcexcm to 1.0 xcexcm in thickness is formed on the nickel-plated layer. Subsequently, a tin or solder plated layer is formed on the copper undercoat layer. After that, reflow treatment or hot dipping of tin or solder is performed on the copper undercoat layer. Thus, a tin-copper intermetallic compound of 0.2 xcexcm to 2.0 xcexcm in thickness is formed on the nickel-plated layer.
However, when such prior art is used for lead frames of tantalum solid electrolytic capacitors, it is absolutely necessary to form a copper undercoat layer for the purpose of forming a tin-copper intermetallic compound layer. Therefore, the cost of the product will become very high, and it is difficult to employ the method from the commercial and industrial points of view.
The present invention is intended to provide a solid electrolytic capacitor which ensures excellent solder wettability for a long period of time due to the plated film having a simplified configuration and is also provided with terminals having excellent solder wettability and heat resisting adhesion, and a method of manufacturing same.
A solid electrolytic capacitor of the present invention comprises:
a capacitor element having a positive electrode leading portion and a negative electrode leading portion;
a positive electrode terminal electrically connected to the positive electrode leading portion; and
a negative electrode terminal electrically connected to the negative electrode leading portion,
wherein the positive electrode terminal and the negative electrode terminal comprise:
one metallic member including at least one selected from the group consisting of nickel, nickel alloy, copper and copper alloy;
a first plated layer, including at least one of tin and tin alloy, disposed on the metallic member; and
an intermetallic compound layer formed between the metallic member and the first plated layer, and
wherein the intermetallic compound layer comprises (i) tin contained in one out of the tin and tin alloy, and (ii) a compound including at least one out of nickel and tin contained in the metallic member.
Preferably, the intermetallic compound is formed by heat reflow treatment of the metallic member disposed on the first plated layer.
In this way, it is possible to obtain a solid electrolytic capacitor which ensures excellent solder wettability for a long period of time due to the plated film having a simplified configuration and is also provided with terminals having excellent solder wettability and heat resisting adhesion, and a method of manufacturing same.